This invention relates to a switching inverter type power supply circuit utilizing resonance.
A conventional dc power supply circuit generally employs a large power transformer with a commercial ac power supply source being connected to its primary winding and a secondary winding side output being rectified and smoothed. Aside from this dc power supply circuit, there are small and light power supply circuits called switching power supply circuits. These switching power supply circuits include switching inverter power supply circuits of a half-bridge type, a full-bridge type and a push-pull type using switching elements such as a transistor and a thyristor.
The conventional switching inverter type power supply circuits, however, produce switching noises because a square wave is used for both operation current and operation voltage. Besides, since these power supply circuits are operated fundamentally at a high frequency, a considerable power loss and resulting heating occur during recovery time of a rectifying diode and a turn-off time of switching transistor.
In the field of the switching power supply circuits, there have been proposals (e.g., Japanese Patent Application Laid-open No. Sho 64-43062 and Japanese Patent Application Laid-open No. Hei 1-91659) to reduce noise and improve efficiency by causing a current waveform or voltage waveform to simulate a sinusoidal wave by utilizing resonance.
In the proposed resonance type switching power supply circuits, however, setting of on-off timings is rather difficult and, when this setting is insufficient, desired improvement of efficiency cannot be achieved. Besides, the proposed resonance type power supply circuits cause either a current waveform or a voltage waveform only to simulate a sinusoidal wave by resonance. Considering that noises produced by the switching circuits include both noises occurring due to voltage and noises occurring due to current, noise reduction obtainable by these switching circuits has apparently certain limitation.
It is, therefore, an object of the invention to provide a switching inverter type power supply circuit of a high efficiency and low noise.